Two kinds of programmable controllers (PC) are generally used: one as an independent unit; and the other as a unit contained in a numerical control apparatus (CNC). Hereinafter, the PC contained in the numerical control apparatus is referred to as a programmable machine controller (PMC).
FIG. 3 is a wiring diagram of a conventional PMC, in which a PMC 20 is contained in a numerical control apparatus 10. A sequence program for controlling the particular operations of mechanical units A 61 and B 62 is stored in the PMC 20 and controls the mechanical unit A 61 through an I/O circuit 31 and the mechanical unit B 62 through an I/O circuit 32 in response to a command.
The operation of some of the mechanical units controlled by the PMC 20 for example, an automatic pallet changer (APC), loader and the like, are independently effected, because a sequence program, for example, controlling these mechanical units can be more easily debugged when they are handled independently.
Nevertheless, when a sequence program for all of the wired mechanical units is stored in the PMC 20, as in prior art, all of the mechanical units must be wired when debugging is carried out.
Further, when another mechanical unit is to be added later, a necessary modification of the sequence program is very cumbersome.